Published Sept. 5, 2025, 1:18 p.m.
Topaz turns blue thanks to X-rays
An international team of researchers, coordinated by the STAR research infrastructure at the University of Calabria, has achieved an unprecedented result: the transformation of colorless topaz into a blue gemstone using exclusively X-rays produced by a synchrotron. The study, published in Physica B: Condensed Matter and available open access on the journal’s website, not only introduces a new technique to the field of gemological treatments but also helps clarify the physical mechanisms underlying this color change.
Topaz is a highly valued gemstone in jewelry due to its exceptional brilliance and hardness. However, most naturally occurring topaz is colorless. To achieve the coveted blue shades, the jewelry industry has relied for decades on irradiation with neutron, electron, or gamma-ray beams. While effective, these high-energy treatments are complex, costly, and require significant safety precautions. The study, however, demonstrates that the same coloration can be achieved with a simpler approach: controlled irradiation using very intense X-rays.
The key point of the discovery concerns how irradiation alters the crystal. When topaz absorbs X-rays, a cascade of secondary electronsis generated that interact with the atomic lattice. These processes create so-called Frenkel defects, in which an oxygen atom bonded to a hydroxyl group (OH) moves from its regular position: a vacancy, or “empty site,” remains in the lattice, while the atom occupies an interstitial site, a space normally unoccupied. It is precisely these imperfections that give rise to color centers, electronic configurations that selectively absorb red light and make the crystal appear blue. In this experiment, a subsequent heating to 260 °C allowed the removal of unstable defects responsible for temporary brownish hues, while the stable color centers remained unchanged, ensuring the permanence of the blue coloration.
The treated samples retained their hue for over three years at room temperature, with no perceptible changes. This is an important result, as it ensures not only the effectiveness but also the long-term stability of the treatment.
“What we have demonstrated,” explains Raffaele Agostino, physicist at the University of Calabria and one of the heads of the STAR infrastructure, “is that the color of a mineral does not depend solely on the intensity of the radiation it is exposed to, but on how the crystal lattice responds by generating defects and stable color centers. This result opens up a new perspective: we are no longer just describing the phenomenon, but beginning to understand its mechanisms, which allows us to envision more targeted and safer treatments for other materials as well.”
“From a gemological perspective,” notes Giuseppe Elettivo, expert gemologist, “this technique opens up very interesting prospects: it allows natural gemstones to be enhanced without altering their chemical structure and without resorting to potentially harmful processes. It means being able to offer the market authentic stones, with stable colors, obtained in a traceable and sustainable way. It is an approach that could become a benchmark for the industry, also in terms of certification and consumer trust.”
Beyond its direct impact on jewelry, the results are also relevant to other fields. The ability to induce and stabilize electronic defects in a crystal could find applications in materials science and advanced optics, where fine control over microscopic properties opens the door to new functionalities.
The experiment was conducted on the DXRL beamline at Elettra–Sincrotrone Trieste, designed to irradiate samples with controlled doses of X-rays and ideal for advanced materials studies. The project, which involved experts in gemology, solid-state physics, and spectroscopy, was coordinated by STAR (University of Calabria) in collaboration with the University of Bari, Graz University of Technology, and Elettra–Sincrotrone Trieste, with the support of the European consortium CERIC-ERIC.
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G.S. Elettivo, M. Ferraro, R. Filosa, A. Nicolino, B. Marmiroli, A. Turchet, R.G. Agostino,
On the role of secondary electrons in the color change of high-dose X-ray irradiated topaz,
Physica B: Condensed Matter, Volume 716, 2025, 417717.
https://doi.org/10.1016/j.physb.2025.417717